Greenhouse Heat Loss
Calculator

It calculates heat loss through the greenhouse's surface area using the U-value (insulation quality) of the glazing material and the temperature difference between inside and outside, since heat loss is directly proportional to both surface area and that temperature gradient.

Single-pane glass or plain polyethylene film has a much higher U-value (poorer insulation) than double-wall polycarbonate or double-glazed glass, so switching glazing types can reduce heat loss by 40-50% for the same greenhouse size without changing anything else.

Yes, higher wind speeds increase the rate of convective heat loss from the greenhouse surface, which is why exposed, windy sites lose heat faster than sheltered ones even at the same outdoor temperature, and why windbreaks are a common way to reduce heating costs.

The calculated heat loss in BTUs per hour at your area's coldest expected temperature tells you the minimum heater output needed to maintain your target indoor temperature during the coldest nights, so undersizing based on average rather than minimum temperatures risks the greenhouse dropping below a safe temperature during cold snaps.

Gaps, poorly sealed vents, and door leaks allow warm air to escape through infiltration regardless of how well-insulated the glazing itself is, and in older or poorly sealed greenhouses, air leakage can account for as much heat loss as conduction through the glazing surfaces.